Metal-porcelain-glass vacuumtight structure



942. P. R. KALISCHER ET AL 2,300,931

METAL-PORCELAIN-GLASS VACUUM-TIGHT STRUCTURE Filed June 28, 1940 Fig. Z.

WITNESSES: INVENTORS Patented Nov. 3, 1942 MElAL-PORCEIAIN-GLASS VAGUUM- TIGHT STRUO'IURE Philip R. Kalischer and Melvin J. Kofoid, Wilkinsburg, Pa, assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 28, 1940, Serial No. 342,932

Claims.

Our invention relates to vacuum-tight structures in which porcelain members must be used closely adjacent to vacuum-tight seals of glassto-metal, and in particular, relates to devices in which bore-silicate glass seals must be positioned closely adjacent to a porcelain which is of a special type which is resistant to thermal shock or sudden and violent changes in temperature.

One object of our invention is to provide a method of constructing vacuum-tight discharge tubes having a gap between two metal members which are insulated from each other by glass.

Another object of our invention is to provide a discharge gap between two metal electrodes which are spaced close together over a portion of the surfaces and which are interconnected with each other by vacuum-tight insulation at points where the seals between the metallic members are protected from arc-over by porcelain shields.

Still another object of our invention is to provide a structure in which a spark gap of small length and large cross-sectional area intervenes between two metallic members which are sealed to each other vacuum-tight by glass which insulates them from each other, and in which the gap is protected from the efiects of arc-over by a shielding member of porcelain or other refractory material which has a coefiicient of thermal expansion materially different from that of the glass. Other objects of our invention will become apparent upon reading the following specification taken in connection with the drawing, in which:

Figure 1 shows a plan view; and

Fig. 2 shows a view in cross section of a structure embodying our invention.

For certain purposes, for example, in lightning arresters, it is desirable to provide two metal terminals between which there intervenes a spark gap of very small length between substantially flat, parallel surfaces, and to be able to completely control the nature and pressure of the gaseous atmosphere within the spark gap. In order to exert the last mentioned pressure control, it is necessary to provide vacuum-tight seals of insulating material connecting the two metal terminals. In the case of such closely spaced electrodes as have been described above, convenience in fabrication and other reasons make it necessary that glass seals between the two terminals be located at points where the opposed terminals are spaced apart more widely than at the above mentioned short gaps. For

spacing and other purposes it is desirable to employ properly disposed rings of porcelain or other thermal-shock resistant material lying closely adjacent to the glass surface within the vacuumtight enclosure.

Where discharge currents of large magnitude have to be provided for, it is necessary to employ porcelain of a type which is especially resistant to sudden and severe heating; such porcelain may be referred to as thermal-shock-resistant.

As is disclosed in Scott Patent No. 2,062,335 assigned to the Westinghouse Electric & Mann-- facturing Company of East Pittsburgh, Pa., it is possible to form very satisfactory vacuum-tight seals between boro-silicate glass such as that shown as G-705-AJ of the Coming Glass Company, Corning, New York, and an alloy comprising substantially 16.3% cobalt, 29.4% nickel, 0.2% manganese, and theremainder iron.

As is disclosed in Bahls Patent No, 2,147,417, assigned to the Westinghouse Electric 8: Manufacturing Company, there are porcelains which have coefiicients of expansion so closely similar to that of boro-silicate glass that the glass may be fused in contact with the porcelain and cooled down to room temperature without difficulty from the two cracking apart. If the shields above-mentioned as lying in close contact to the boro-silicate glass in the above-mentioned spark gap structures were of the porcelain mentioned in the Bahls application, no difliculty would arise from the contact of the porcelain with the glass while the latter is being fused to form the seal. However, this porcelain is not of the thermal-shock-resistant type, but the latter has a coefilcient of expansion which is much smaller than that of boro-silicate glass. As a result, when shock-resistant-porcelain is used in the spark gaps above mentioned, the glass is cracked by its difference of expansion from the porcelain in contact with which it lies as soon as the seal cools down.

In accordance with our invention, we have provided a mica washer intervening between the surface of the porcelain and the adjacent surface of the boro-silicate glass and find that such provision prevents the cracking of the glass when the seal cools.

In accordance with the foregoing principles, Figs. 1 and 2 show a cup-shaped member I of metal which is adapted to form one terminal of a spark gap. A second cup-shaped member of metal, slightly smaller in outside diameter, forms the other terminal. The two metal members I and 2 are slightly dished in a drawing press to form adjacent flat surfaces 3 and l in their centhereto, being made of the above-mentioned alloy.

The properties of the above mentioned metal and steel are such that they can be readily welded to each other.

In assembling the spark gap, an annular porcelain member 'I is placed in the bottom portion of the member I and a washer 8 of mica is laid on top of the member I.

We have found it necessary, in order to prevent the formation of bubbles and other defects at the junction of the mica with the glass about to be described, to bake mica at about 1100 C. or from 10 to 16 hours in an atmosphere of dry nitrogen. The washer 8 and the member I are so proportioned to the metal members I and 2' that the separation between the two fiat surfaces adjacent the center of the latter produces the desired length of spark gap. The metal terminal 2 is then placed on top of the washer 8 in a centralized position thereon and the ring-shaped trough between members I and 2 and above the washer 8 is then filled with glass, for example, G-705 AJ of the Coming Glass Works, Corning, New York. The assembled structure is then heated to beyond the melting temperature of the glass, for example, to around 1000" C.- As a result, the glass becomes liquefied and forms vacuum-tight seals with the rims and 6. The device is then cooled down to room temperature at the rate of approximately 2 C. per minute. When this is done, it is found that the seals between the members 1 and 2 remain intact.

The foregoing is a brief outline of the process used in carrying out our invention. Certain detailed steps of the method, however, will be pointed out in the following tabulation.

1. Decarburize the alloy parts by heating in wet hydrogen for 5 to hours at about 900 C.

2. Oxidize the alloy parts by heating in still air for 3 min. at 800 C.

3. Dehydrate mica by heating in dry nitrogen for 10 to 16 hours at 1100 C.

4. Assemble equipment.

5. Heat in oxygen-free dry nitrogen to 1050 C. This may be done rapidly since the porcelain is thermal-shock-resistant.

6. Hold at 1050 C. for 30 minutes.

7. Cool to 500 C. and hold 1 hour to relieve strains.

8. Cool to 450 C. and hold 3 hours or more.

9. Cool to room temperature.

To give a specific description ofone embodiment of our invention with the understanding that this description is merely illustrative and not a limitation upon this disclosure. The cup I may be formed of steel approximately 0.060 in. thick to have an outside diameter at the rim of 3%, in. and the rim having a width of in. The

central fiat area 3 has a diameter of about 1% in.

The member 2 may be formed of steel about .060 in. thick and 2% in. in the outside diameter. It is welded to a ring of the above mentioned alloy about .060 in. thick and in. wide. The fiat depressed portion at the center of the member 2 is approximately 1% in. in diameter. The distance between the fiat surfaces 3 and 4 is to be approximately 4, in. The member I is approximately 3 in. in outer diameter and 1% in. in inner diameter and A in. thick. As an example of a thermal-shock resisting body a porcelain containing crystallized corderite may be mentioned. The

mica washer 8 may be about .020 in. in thickness,

3 in. outside diameter and 2 in. inside diameter.

While we have described our invention in connection with the manufacture of a spark gap of a particular type, it will be recognized that the expedient forming glass-to-metal seals along the glass is necessarily closely adjacent to refractory material having a coefiicient of expansion substantially different from that of the glass is applicable for many other purposes which will be evident to those skilled in the art. The principles of our invention are accordingly broader than the particular instants of our use which have been described in the foregoing and other uses of these principles will be evident to men skilled in the art.

A circuit protective system adapted to employ spark-gaps of the type herein disclosed is described and claimed in Berkey and Slepian an,- plication Serial No. 358,634 filed on or about September 27, 1940. a I

We claim as our invention:

1. In combination, a porcelain body, a pair of metal members lying closely adjacent thereto, a glass filling intervening between said metal members and fused vacuum-tight thereto and a mica sheet interposed between the glass and the porcelain.

2. The method of forming a joint between a pair of metal members, said joint being located closely adjacent to a porcelain body, which comprises baking a mica sheet in the region of 1100 C. for a period of the order of 10 hours in a dry nitrogen atmosphere, interposing said sheet between said porcelain body and said joint and depositing and fusing a glass filling vacuumtight between said metal members.

3. In combination, a porcelain body, a pair of metal members lying closely adjacent to said porcelain body, a cobalt-nickel-iron alloy surface on a portion of each of said metal members, a

. filling of boro-silicate glass fused vacuum-tight to said members, and a mica sheet interposed to separate the glass from the porcelain.

4. In combination, a porcelain body, a pair of metal members lying closely adjacent to said porcelain body, a surface on each of said metal members comprising from 16 to 18% cobalt, 28 to 30% nickel, and the remainder iron, on a portion of each of said metal members, a filling of boro-silicate glass, comprising substantially! 67.3% S102, 1.73% A1203, 5.5% alkali oxides, 24.6% B203, fused vacuum-tight to said members, and a mica sheet interposed to separate the glass from the porcelain.

5. An electrical discharge gap comprising a pair of metallic terminals in the form of nested shallow cups, a porcelain ring sandwiched between said terminals, a mica washer adjacent said ring, the diameter of one of said cups being smaller than the other, thereby leaving an annular gap between the cups, surfaces of a cobalt-nickel-iron alloy along the adjacent edges of said gap. and a filling of boro-silicate glass fused to said surfaces to form a vacuum-tight seal between said terminals.

PHILIP R. KALISCHER. MELVIN J. KOFOID. 

